paghat wrote:
...If shells are really to be a useful soil ammendment, they'd need to be
crunched up rather than tossed about entire. Crunched up shells can also
be severely alkalinizing, as their composition is identical to limestone,
& should be used with all the plusses & minuses in mind for using lime...
...I have seen recommendations that crushed oyster & clam shells can be used
as a compost additive to help retard the possibility of souring..


Crushing shells increases their surface area and therefore soil
reactions will take place faster. Any pH changes that the shells will
force will occur more quickly. By the same token, the effect of the
shells will be shorter-lived in the soil, so you will have to apply them
on a regular basis.

If your soil does not need much lime, you might consider using the whole
shells, since they will react slowly and will last longer.

If your soil does not need any lime, the whole shells will not produce
much of an impact.

If your soil is alkaline, the shells will not help and may be a
detriment.